(1) Field of the Invention
This disclosure relates to a thin film transistor substrate and a fabricating method thereof.
(2) Description of the Related Art
Liquid crystal display (“LCD”) devices include a first substrate, which includes a thin film transistor (“TFT”), a second substrate disposed opposite the first substrate, and a liquid crystal layer interposed between the first and the second substrates. The liquid crystal layer may have anisotropic dielectric properties. LCDs can display images by adjusting the amount of light transmitted through the liquid crystal layer. The behavior of a liquid crystal layer can be controlled by a voltage difference applied to a common electrode and a pixel electrode. TFTs are widely used as a switching device in an LCD.
TFTs are switching devices, which include a gate electrode, a drain electrode, a source-pixel electrode, and an active layer. When a voltage higher than a threshold voltage is applied to the gate electrode, current can flow between the drain electrode and the source-pixel electrode. The active layer of a TFT can include amorphous silicon (“a-Si”), or polycrystalline silicon (“poly-Si”).
In order to produce a larger and higher resolution display, a thin film transistor substrate with a high field effect mobility would be desirable. Additionally, in order to fabricate a larger display with an integrated driving circuit in the panel, it would be desirable to decrease a line resistance and a parasitic capacitance. It would also be desirable to increase the field effect mobility of a thin film transistor, which can be used as a switching device or a driving device. Recently, a microcrystalline silicon layer, which can be disposed by a deposition method, and a polycrystalline silicon layer, which can be disposed by crystallization of an a-Si layer, have been investigated for fabrication of a thin film transistor with a high field effect mobility. However, current microcrystalline thin film transistors show low field effect mobility, and an improved deposition apparatus would be desirable in order to improve the electrical properties of current microcrystalline thin film transistors. A polycrystalline silicon thin film transistor can have acceptable field effect mobility, but a commercially available apparatus for the crystallization process is costly, and a uniformity of the field effect mobility of a commercially available polycrystalline silicon thin film transistor is poor due to the crystallization mechanism.
To resolve these and other problems, application of an oxide semiconductor as an active layer in a thin film transistor has been considered. Because the field effect mobility of an oxide semiconductor thin film transistor can be greater than ten times higher than that of an a-Si TFT, a pixel chargeability can be increased and integration of a driving circuit, including the oxide semiconductor thin film transistor substrate, would be desirable. Moreover, a fabrication process of an oxide semiconductor TFT may be similar to that of an a-Si TFT. However, current processes, which are performed after disposing an oxide semiconductor, such as a plasma process or a wet process, or electrical stress, can degrade one or more electrical properties of the oxide semiconductor thin film transistor.